What type depth finder do you have?
I have a similar boat (Paceship PY26) , which when I bought it had an old thru-hull depth sounder that always seemed to think it was in the Marianas Trench....
I installed a Standard Horizon #35 with an In-hull Puck transducer(Airmar P79 which is the kind that sits in a base filled with mineral oil). The display unit can be flush-mounted or on a small bracket that comes with it. I put the transducer base in with 3M-5200, but the transducer itself is removable. The system works great, and the whole thing ran about $200 at West Marine.